1. Field
The present disclosure relates generally to focal plane arrays. More particularly, the present application is directed to a method and apparatus for a focal plane array with serial analog to digital converters with dynamically reconfigurable conversion resolution.
2. Background
A focal plane array is an image sensing device consisting of an array of light-sensing pixels at a focal plane of a lens. Focal plane arrays are used for a variety of purposes, such as, without limitation, imaging purposes, spectrometry, wave-front sensing, and scanning arrays. Focal plane arrays are frequently used in video cameras and other imaging devices.
For example, video cameras in machine vision applications frequently utilize focal plane arrays. These video cameras can generate vast amounts of digital streaming data. The data rate in units of bits per second is determined by the number of frames per second that the camera can capture, multiplied by the number of pixels per frame, and multiplied by the number of bits per pixel. The number of bits per pixel is determined by the data width of the analog to digital converter used by the focal plane array, and is fixed by the hardware to a value that gives the maximum gray scale resolution that the focal plane can offer above the noise floor of the focal plane array.
It is often desirable to acquire data at the maximum possible frame rate of the camera for applications such as closed loop optical tracking and low latency image analysis. Although the number of bits per pixel is fixed, the frame rate must be slowed down so that the amount of data generated by the camera does not exceed the bandwidth of the data link.
However, many image processing applications do not require the full gray scale resolution. Often, in machine vision applications such as object counting, edge detection, or optical character recognition, the video image is “binarized” to only one bit of information per pixel. This operation is usually performed on the data by a video processor after the data has been transmitted through the data link. The full bandwidth of the data link is required for the data transfer even though only a small fraction of the data is actually used. If the gray scale resolution of the camera is not altered for applications that do not require the full gray scale resolution, unnecessary data bits are generated which results in a slower and less efficient process than is necessary.
High frame rate cameras typically use high speed serial data links. For example, the CamLink standard allows the use of multiple parallel data channels to accommodate the high bandwidth requirement. However, the use of multiple parallel data channels requires more interconnections and more electronic drivers and receivers. In addition, synchronizing the parallel data streams can be complicated. The video data from the camera may be processed using algorithms that do not require the full gray scale resolution of the camera. This operation is typically performed by a microprocessor after the full gray scale analog to digital converted data is transferred through the data link. The added bandwidth of the data link is necessary for the data transfer even though most of the information is unused.
If the bandwidth of the data channels is still insufficient to accommodate the streaming data from the focal plane array, it is a common practice to reduce the number of pixels per frame by windowing the focal plane array to a specific region of interest (ROI). This windowing function is typically performed by the readout electronics of the focal plane array. If the focal plane array is operated in a windowed mode, the frame rate may be increased to utilize the maximum available bandwidth, but any visual stimulus occurring outside the region of interest will be missed.
In another solution, cooled infrared cameras bring the analog pixel data to analog to digital converters that are located away from the temperature controlled area. However, the physical size and wiring requirements of these external analog to digital converters associated with cooled infrared cameras may result in significant disadvantages.
Moreover, existing analog to digital converter topologies may consume a significant amount of power and may occupy a large amount of die area. This may make the integration of analog to digital converters into focal plane array readout electronics difficult or impractical. In addition, infrared focal plane cameras are typically cooled, so the added heat generated by the analog to digital converters generally necessitates their placement away from the temperature controlled area. Thus, current solutions frequently do not integrate analog to digital converters into focal plane array readouts.
Accordingly, there is a need for a method and apparatus for integrating analog to digital converters into the readout electronics of an infrared focal plane array while minimizing the analog signal path and reducing noise susceptibility, which overcomes the problems discussed above. Embodiments of the disclosure are intended to satisfy this need.